This application claims Paris convention priority of German Patent Application 10015068.3 filed on Mar. 25, 2000, the complete disclosure of which is hereby incorporated by reference.
The invention concerns a method of producing magnetic resonance images, wherein a (n+1) dimensional k space is scanned, comprising an imaging pulse sequence with at least one RF excitation pulse followed by at least one RF refocusing pulse, wherein an incomplete, complex spin echo signal Sx is measured and digitized in one part of the read-out interval [t0xe2x88x92xc2xdta, t0+xc2xdta] by means of a quadrature detector, which comprises a central part about the center (t=t0) of the spin echo signal in the time interval [t0xe2x88x92xcex5, t0+xcex5] with n(n=0, 1, 2, . . . ) phase encoding gradients before the read-out interval.
A method of this type is disclosed e.g. in U.S. Pat. No. 4,851,779 or U.S. Pat. No. 4,780,675.
In conventional methods of magnetic resonance for producing sectional images, the duration of scanning of the region of interest is that long that movements of the body region observed produce changes which impair the image quality. Due to the number of measuring sequences required for an examination, the total examination time could be between xc2xd and 1 hour. Long examination times are not desired, in particular not for patients.
To reduce the scanning time, it was tried to scan only incomplete k spaces. The incomplete k spaces are completed by complexly conjugated reflection of the detected data. To carry out this reflection, the entire (incomplete) data set must have been recorded before processing start. Reconstruction requires additional intermediate results in the form of phase images or images of a partial echo.
U.S. Pat. No. 4,851,779 discloses collecting an incomplete data set of three-dimensional magnetic resonance data and storing it in a memory. The incomplete data set is complete in a first and second direction, however, incomplete in a third direction. The detected data set includes data along the third direction between xc2x1n central values and the half of the other values. One-dimensional inverse Fourier transformations are applied in the first and second direction to obtain an intermediate data set. A phase correction matrix or a plurality of phase correction vectors p(r) is produced from the intermediate data set and stored in a phase correction memory.
A symmetrical data set is produced as conjugated complex from the intermediate data set. The intermediate data set and symmetrical data set are inversely Fourier-transformed in the third direction (fa, fs), then the vectors of both image matrices are corrected with the corresponding phase vectors and combined into a line of a resulting three-dimensional image.
Disadvantageously, all data must be recorded in such a method before carrying out Fourier-transformation and intermediate results and phase matrices must be stored.
U.S. Pat. No. 4,780,675 discloses collecting an incomplete set of magnetic resonance image data and storing it in a memory. The incomplete data set comprises a central or first data set and an additional or second data set. A roll-off filter and Fourier transformation is applied to the central data set to obtain a normalized phase image. The first and second data set are Fourier transformed and phase corrected by multiplying with the conjugated complexes of the corresponding phase value. A third data set is produced by determining the conjugated complexes of the second or additional data set. The third data set is Fourier transformed and multiplied with a corresponding value of the phase image to produce a second phase-corrected image representation. The first and second corrected image representations are added and stored in an image memory.
Also in this method, all data must be recorded and stored before further processing. Additionally, the central data set is filtered before producing the phase diagram.
It is therefore the underlying purpose of the present invention to provide a method for faster imaging with substantially constant image quality without increased technical effort.
In accordance with the invention, this object is achieved in a simple but effective fashion in that the digitized, incomplete, complex spin echo signal Sx is completed through adding zeros for the entire read-out interval and the central part is weighted with a function which is substantially anti-symmetrical about the point t=t0 and has an amplitude of xc2xd at t0 and subsequently is Fourier transformed for generating a Fourier-transformed signal.
In this method, only slightly more than half of the data of one dimension of a k space is detected. The remaining data is replaced by zeros. At least half of the k space must be detected in the reading direction to determine the position of the center of the spin echo signal and to prevent a loss in resolution.
The inventive method has the advantage that the image is continuously reconstructed and possibly corrected directly after recording each k space line. In particular for resonance images with more than one dimension, this proves to be advantageous since the directly following reconstruction saves considerable time.
After Fourier transformation, the phase can be corrected which produces real images with the same quality. The required phase corrections are known already before data recording. They are determined in a pre-scan. The data is reconstructed parallel to data recording.
By calculating the magnitude of the complex values and the omission of phase correction, image quality is considerably improved.
The article xe2x80x9cFaster MR Imagingxe2x80x94Imaging with Half the Dataxe2x80x9d Society of Magnetic Resonance in Medicine, 1985, pages 1024-1025 describes using half of the phase encoding steps for image reconstruction, whereas the other half is empty and use the real part of the complex image formation for image representation. To prevent ringing artifacts, a roll-off filter must be applied which requires adding some additional phase encoding steps.
To improve the prior art approach, an alternative method variant which utilizes the same inventive basic idea, concerns a method of producing magnetic resonance images wherein a (n+1) dimensional k space is scanned, comprising an imaging pulse sequence with at least one RF excitation pulse in a first step, followed by at least one refocusing pulse, wherein in at least one part of a read-out interval [t0xe2x88x92xc2xd ta, t0+xc2xd ta) at least one part of a complex spin echo signal Sx is measured and digitized, by means of a quadrature detector, which comprises a central part about the center t=t0 of the spin echo signal in the time interval [t0xe2x88x92xcex5,t0+xcex5] having at least one phase encoding gradient before the read-out interval and wherein in subsequent steps, the phase encoding gradient is systematically incremented and the k space is incompletely scanned in the direction (ky) of the phase encoding gradient such that for each relative point in time in the read-out interval in the phase direction (ky) an incomplete signal Sy is obtained having a portion central about ky=0 wherein for each relative point in time in the read-out interval, the digitized incomplete complex signal Sy is completed through adding zeros along ky and the central part having a function which is substantially anti-symmetrical about the point ky=0 having an amplitude of xc2xd at ky=0 is weighted and subsequently Fourier-transformed for producing a Fourier-transformed signal.
Also in this case, only slightly more than half of the data must be read in the direction ky and data processing and simultaneous reading of the remaining data is possible. This permits on the one hand more rapid imaging and saves on the other hand memory capacity.
In an advantageous further development of the two above-described variants of the inventive method, the Fourier-transformed signal is phase-corrected to clearly separate the real and imaginary parts of the result.
A further preferred embodiment of the inventive method is characterized in that for determining the coefficients of the zero and first order of the phase correction of the Fourier transformed of the spin echo signal, the spin echo signal is recorded without previous influence of a phase encoding gradient and its center is determined by means of an algorithm which takes into consideration the central symmetry thereby utilizing the central symmetry of the spin echo signal. This allows more rapid determination of the center by interpolation methods known per se.
In a further advantageous method variant, the weighting function is a linearly rising or descending function of time which is easy to realize technically. Preferably, the weighting function thereby drops monotonically from one to zero in the interval [t0xe2x88x92xcex5,t0+xcex5]. This compensates over-weighting of the central points during Fourier transformation.
In a particularly preferred further development of the inventive method, the weighting function is a constant with the value xc2xd. This method variant is particularly easy to carry out. It is principally feasible to use any function having an amplitude of xc2xd at point t0 and is anti-symmetrical to this point.
It is particularly advantageous to start Fourier transformation before scanning of the k space is finished which saves a lot of time, mainly if multiple dimensions are scanned.
A further preferred method variant is characterized in that a dephasing portion of a read gradient is switched before or directly after the RF refocusing pulse and a rephasing portion of the read gradient is switched at least during the interval [t0xe2x88x92ta/2, t0+xcex5] or [t0xe2x88x92xcex5, t0+ta/2] wherein under the effect of the rephasing portion of the read gradient, part of the complex spin echo signal Sx is measured. The use of a read gradient eliminates, in principle, one dimension of the k space, namely that of the chemical shift. Thus, the data of one dimension can be recorded in one single read-out process which allows particularly fast detection of data.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and explained in more detail by means of embodiments.